When Escherichia coli is subjected to carbon source shift-down, protein synthesis is inhibited and a time-dependent disaggregation of polyribosomes and concomitant accumulation of monosomes is observed. These monosomes are "complexed" rather than "free" ribosomes by several operational criteria yet they carry no nascent polypeptide chains. We propose to test the hypothesis that these monosomes are blocked translation initiation complexes which represent a physiologically significant mechanism for regulating protein synthesis. We will test for N-formylmethionyl-tRNA in the monosomes by hydrolysis followed by identification of the released amino acid and identification of any tRNA's by their ability to be aminoacylated in vitro. The presence of messenger RNA in the monosomes will be confirmed by DNA-RNA hybridization and by electron microscopy. The monosomes will be tested for the ability to carry out polypeptide synthesis in vitro. The fact that active polyribosomes still appear under conditions of gross monosome accumulation suggests that some specificity may be involved with respect to the ribosomes or the species of mRNA which are selected for translation. This possibility will be examined by hybridization- competition of the mRNA's from the monosomes and polysomes and by comparing ribosomal proteins from the two classes. The accumulation of monosomes and free ribosomes will be measured under a variety of physiological conditions, including carbon source shift-town, carbon source starvation, inhibitor-induced shift-down, nitrogen starvation and amino acid deprivation.